Project Summary A deeper understanding of the molecular mechanisms regulating hematopoietic lineage specification is critical for developing improved therapeutics for disorders that affect red blood cell and platelet abnormalities. Currently, we do not know the mechanisms that influence the fate decisions of the megakaryocyte-eryhtroid progenitors (MEP) that can differentiate down either the red blood cell or megakaryocyte lineage. The overall goal of this proposal is to identify the mechanisms by which the lineage fate is determined in these bipotent progenitors. Progress in determining how bipotent cells become committed has been hampered in part due to lack of ability to identify and enrich for bipotent cells that are at this critical stage. The Krause laboratory has recently addressed this barrier to progress by enhancing, and then using, an in vitro functional assay for individual bipotent MEP to develop improved approaches to enrich for the cells. Our preliminary data using these MEP strongly support the hypothesis that more rapid cell cycling causes an MEP to become biased toward the erythroid over the megakaryocytic lineage. These data include 1) single cell RNA deep sequencing to prove that the enriched cells represent a unique progenitor population that is not fully committed to either of its potential downstream fates and to provide hypothesis-generating data on potential mechanisms of MEP fat determination; 2) determination of small molecules that influence fate decisions; 3) validation of an approach to test knockout of specific genes that affect the fate decision (e.g. MYB); 4) CFSE assessment of changes in proliferation and accompanying cell fate biases; 5) validation of longterm timelapse microscopy from single cells to colony formation to assess cell cycle timing and fate determination; and 6) use of a novel in vivo cell cycle timer reporter. Based on these extensive preliminary data, we propose to: 1) test the hypothesis that cell cycle speed plays a critical role in the MEP fate decision; 2) dissect the molecular mechanisms underlying the MEP fate decision; and 3) test the hypothesis that the elevated platelet counts in humans and mice with iron deficiency anemia are due to a biased MEP fate decision. The results of these studies will contribute to our understanding of fate regulation of normal hematopoietic progenitor cells in mice and healthy human donors, and will provide important insights relevant to the pathogenesis of common treatment-refractory hematopoietic diseases including iron refractory iron deficiency anemia and bone marrow failure. Clinical applications also include enhancement of our ability to produce RBCs and platelets in vitro for transfusion for anemia and thrombocytopenia.